Numerous processes are known for the production of atmospheric gases in particular, oxygen, by means of a cryogenic air separation unit (ASU) for which the feed air compressor is mechanically linked to a combustion gas turbine. Among these are U.S. Pat. Nos. 4,224,045 and 3,731,495.
The escalating costs of energy have intensified research in the field of alternate energy sources. One result of this quest is the recently developed Integrated Gasifier Combined Cycle (IGCC) power plant. Using a mixture of coal and oxygen (where, typically, the purity of the oxygen will be higher than 80 vol % oxygen), the IGCC produces energy--electricity.
Because the operation of such a plant depends on consumer demand for electricity, the input of the plant, specifically oxygen, needs to vary along with the electricity demand. Unfortunately, a problem is created by integrating the ASU (for producing oxygen) with the IGCC having a combustion gas turbine as is taught in U.S. Pat. No. 4,224,045.
In an IGCC that is mechanically linked to an (integrated) ASU, the feed air for the ASU is compressed by a gas turbine. The operation and output of the gas turbine depend on the exhaust gas from combustion of the gasifier product and, in part, from the low pressure gaseous nitrogen product of the ASU. The problem arises because the normal mode of operation for an IGCC is not static. As mentioned, an IGCC is usually required to ramp in response to varying demands for electrical power. By ramping the operation of the gasifier, an operational effect is seen in the combustion gas turbine which in turn will mean variations in the pressure of the compressed feed air to the ASU. The ramping of the IGCC means either an increased or decreased need for products from the ASU, in particular, the quantities of oxygen needed for the gasifier operation. Also, it is important that during increased or decreased production by the air separation unit, the purity of the products remain constant.
However, before the advent of the IGCC, ASU's did not have to vary their production as severely as the operation of an IGCC requires, and they were designed accordingly. To illustrate the problem, during a ramp down of the ASU less product is needed, yet liquids in the distillation columns are flashing as the air supply pressure decreases tending to generate more product (this is contrary to the customer's requirements). Also, the flashing liquid is oxygen rich which could potentially degrade the nitrogen product purity. Thus, the problem: how to control the ramping of an air separation unit which has a varying compressed feed air pressure, varying demands for oxygen and strict purity requirements.